1. Field of the Invention
The present invention relates to ferroelectric thin-film devices and methods for making the same, and particularly relates to a ferroelectric thin-film device, which is applicable to not only capacitors for DRAM (dynamic random access memory) and ferroelectric RAM but also pyroelectric elements, microactuators, thin film capacitors and compact piezoelectric elements, and relates to a method for making the same.
2. Description of the Related Art
Recently, the formation of epitaxial thin films on single crystal substrates has been intensively studied regarding lead-based perovskite compounds, such as PbTiO3, (Pb,La)TiO3 (Pb1xe2x88x92xLaxTiO3, hereinafter abbreviated as PLT), Pb(Zr,Ti)O3 (PbZryTi1xe2x88x92yO3, hereinafter abbreviated as PZT), (Pb,La)(Zr,Ti)O3 (Pb1xe2x88x92xLaxZryTi1xe2x88x92yO3, hereinafter abbreviated as PLZT) and Pb(Mg,Nb)O3 (PbMgzNb1xe2x88x92zO3, hereinafter abbreviated as PMN). This is because when a lead-based perovskite compound having large residual polarization, such as PZT or PLZT, is epitaxially grown, spontaneous polarization can be unified in one direction so that larger polarizability and switching characteristics are achieved, and application to high-density recording media will become promising.
Uses in which such spontaneous polarization is unified in one direction along the thickness, however, requires a metal-ferroelectric-metal (MFM) structure in which the ferroelectric thin film is sandwiched between conductive (electrode) layers on a substrate.
Generally, when a thin film of a lead-based perovskite compound is formed, lead is likely evaporated from the deposited thin film during its formation. As a result, the composition of the thin film often deviates from the intended composition. In order to cope with the problem, lead is supplied in an amount which is larger than the stoichiometric ratio in raw materials in the method for preventing such a change in composition. However, according to this method, excess Pb is oxidized to form PbO. Since the lattice constant of PbO is substantially equal to the lattice constant of the lead-based perovskite compound, PbO also is epitaxially grown under the conditions for epitaxial growth of the lead-based perovskite compound. Thus, the epitaxial growth of the lead-based perovskite compound is inhibited, resulting in deterioration of crystallinity and the morphology of the surface.
The present invention can solve the aforementioned problem and provides a ferroelectric thin-film device having a ferroelectric thin film of a perovskite oxide with high orientation and a method for producing the ferroelectric thin-film device.
The ferroelectric thin-film device comprises a single crystal substrate; a conductive thin film formed on the single crystal substrate; and an oriented ferroelectric oxide thin film having a perovskite structure formed on the conductive thin film. The oriented ferroelectric thin film comprises a first layer having a composition changing from the interface with the conductive thin film in the thickness direction and a second layer having a constant composition formed on the first layer. The composition of the first layer and the composition of the second layer are substantially the same at the boundary between the first layer and the second layer. There may be no distinct interface between the layers.
The oriented ferroelectric thin film is preferably a Pb(Zr,Ti)O3-based thin film, and the Zr content in the first layer gradually increases from the interface with the conductive thin film in the thickness direction, while the Zr content in the second layer is constant.
According to the present invention, using conductive materials and ferroelectric materials having similar lattice constants, a conductive thin film is formed on a single crystal substrate and a ferroelectric thin film including a first layer having a changing composition and a second layer having a constant composition is formed thereon. The resulting ferroelectric thin film has high orientation and extremely reduced crystal defects. Thus, original characteristics of the materials are maintained. Expected applications of the ferroelectric thin-film device are pyroelectric devices, microactualtors, thin film capacitors and compact piezoelectric devices, in addition to DRAM and FeRAM.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.